1. Field of the Invention
The present invention relates to image capture systems (e.g., digital camera) for capturing and processing object images.
2. Description of Related Art
Conventional image capture systems (e.g., digital still cameras) typically include various components, as shown in FIG. 11. In FIG. 11, an overall control CPU (central processing unit) 116 detects a change in state of a camera operation switch (not shown) having a main switch and a release switch. The camera operation switch, which is operated by a user, enables power supply to the respective circuit blocks.
An object image within the range of a photographic image plane is formed on an image sensor 100 (CCD (charge-coupled device) by a main photographic optical system 118. An output signal from the image sensor 100 is converted into a digital signal for each pixel by a CDS/AGC/AD (correlated double sampling/automatic gain control/analog-digital) circuit 103.
The image sensor 100 generates an image signal when driven in a predetermined manner by the output of a driver circuit 101. The driver circuit 101 is for horizontal and vertical driving of each pixel based on a signal from the timing generator (TG/SSG) 102, which determines drive timing for the entire image capture system.
The output signal from the CDS/AGC/AD circuit 103 is applied to a correction block 104. The correction block 103 performs correction for shading caused by a combination of the image sensor 100 and the main photographic optical system 118, and performs correction to remove pattern noise inherent in the image sensor 100.
The outputs of the correction block 104 are sequentially stored, as frame data, in a buffer memory 106 via a front memory controller 105, so that images captured with a continuous shooting operation of the camera are temporarily stored.
The front memory controller 105 operates on the basis of a signal from a timing control block 107, which operates in synchronism with the timing generator 102. Accordingly, the front memory controller 105 operates in synchronism with the image sensor 100.
When at least one frame image has been captured, captured image data stored in the buffer memory 106 is temporarily transferred to a work memory 111 via a rear memory controller 108 under the control of the front memory controller 105.
The rear memory controller 108 operates on the basis of a signal from a timing control block 109, which also operates in synchronism with the timing generator 102.
Then, image data stored in the work memory 111 is read out and applied to a color processing block 112, which is connected to a bus A connected to the work memory 111. The color processing block 112 performs the so-called picture adjustment, such as color interpolation and color matrix correction, to convert the image data into RGB data values and further into YCrCb data values. The color processing block 112 then stores the processed image data in the work memory 111.
The image data re-stored in the work memory 111 is then read out and applied to a JPEG processing block 113. The JPEG processing block 113 compresses image data according to a predetermined compression format and stores the compressed image data in a card memory 115 (usually, a non-volatile memory such as a flash memory) via a card controller 114.
The user can select a viewing mode of the image capture system in which images represented by captured image data can be viewed. In the viewing mode, compressed image data stored in the card memory 115 is read out and is decompressed by the JPEG processing block 113 into normal image data for every pixel. The resultant image data is then transferred to the work memory 111 so as to externally display an image represented by the image data on a monitor display device (not shown).
To control the camera, the overall control CPU 116 executes instructions in accordance with instruction codes stored in an instruction memory 117, which is connected to a bus B connected to the overall control CPU 116. For example, the overall control CPU 116 controls driving of the main photographic optical system 118 via a lens control portion 119 and stores various items of information attached to image data in the memory card 115 via a communication I/F (interface) 110.
Japanese Laid-Open Patent Application No. 2000-253305 discloses an example of the aforementioned system.
When continuous shooting occurs in digital cameras, frame images obtained are gradually stored as raw data in a buffer memory. Then, the stored frame image data are sequentially transferred to a rear processing block, where color processing and JPEG processing are performed. The readout rate of an image sensor can be significantly increased to capture 8 frames or more per second, which is desirable (equivalent to that of silver-halide cameras used by professional photographers).
There are a number of proposed methods for increasing the readout rate of an image sensor. In one method, the frequency of a driving clock signal produced by a timing generator is heightened to simply increase the driving speed of a driver. In another method, image data are simultaneously read out from two or more output terminals of an image sensor, and subsequent processing operations up to storing in a buffer memory are performed in parallel at a plurality of lines corresponding to the number of output terminals of the image sensor.
In the conventional digital camera described above, image data stored in the buffer memory 106 is transferred to the work memory 111 via the memory controllers 105 and 108. The image data stored in the work memory 111 is sequentially transferred to the color processing block 112 and is then converted into RGB image data or YCrCb image data in a predetermined format. The converted image data is sequentially temporarily stored again in the work memory 111. Then, the image data re-stored in the work memory 111 is transferred to the JPEG processing block 113. In such a flow, processing operations such as readout operations are performed via the bus A connected between the respective processing blocks so that bus A becomes inundated with traffic.
Furthermore, when image data compressed by the JPEG processing block 113 is stored in the card memory 115, the writing rate of the card memory 115 itself becomes a bottle neck. In this case, the speed of the work memory 111 for outputting image data processed by the JPEG processing block 113 will inevitably decrease. As a result, a large amount of data accumulates in the work memory 111.
Consequently, even if the frame-advancement speed of the camera is increased, the low speed of the rear processing operation inhibits continuous shooting operation at high speed. Thus, unless the capacity of the buffer memory 106 or the work memory 111 is considerably increased, the camera cannot release the shutter immediately after the continuous shooting operation.